If you're experiencing this difficulty you need to assess what effort you are making to help your dog. It's important to expose our shy and/or insecure dogs to everything we can. Although, it's equally important to limit these exposures so as the dog can SLOWLY become accustomed to the stimulus. For example, if you feel your dog is skiddish around toddlers, try leash walking him/her near a playground. As you casually pass by, slip Fido a treat and offer calm praise. Slowly increase the exposure to longer periods making sure you don't overstimulate your dog. As your dog exhibits more accepting attitude you can increase your level of praise as the reward plus he/she will know this is pleasing you.
They need to absorb said exposures and if we keep them "positive" we are reinforcing the calm and "business as usual" attitude for the dog. Another example might be slippery floors. Years ago when I was training my police dog Lazer, he almost was dismissed from K-9 school because he was terrified of shiny floors! You cannot have a search dog that won't step inside a building to find a bad guy because of floor style. So I exposed Lazer to numerous types of flooring. I made sure that it was positive by playing with him on the foor chasing a kong ball. It was slow at first because of his apprehensiveness. Yet over time, as I increased the duration and intensity of play, Lazer became conditioned or accustomed to surfaces and accepted them as norm. At one point I graduated our effort to include the ice surface of a local hockey rink. He ended up having so much fun I had to stop because I didn't want him to tear a ligament. Therefore, remember to always keep it safe and be mindful of your surroundings--You are your dog's Leader and although we want him/her to protect you, we must also protect them. As they say there's no "I" in team. And a canine team is made up of both dog and its handler. Show confidence, a calm and respectful attitude in your teachings and your dog will be the shining pupil you're looking for.